There is a demand for establishment of a high-speed optical transmitter. For this reason, a development of an optical transmitter adopting multilevel modulation method used in a wireless system is moving toward practical use. An optical transmitter adopting polarized-wave multiplexing technology and digital coherent receiving technology attracts an attention as a method for establishing a transmission system having large capacity and long distance.
In the polarized-wave multiplexing method, two data streams are transmitted with use of two polarized-waves crossing at right angle having an identical wavelength. Thus, the polarized-wave multiplexing technology reduces a modulation speed to half, and contributes to characteristics improvement of an electrical signal generation circuit, lower cost, downsizing, lower power consumption and so on. And, influence caused by quality degradation such as dispersion on a transmission path is reduced. Thus, characteristics of an optical transmitter are improved.
An optical transmitter generating a polarized-wave-multiplexed signal has a modulator with respect to each polarized-wave signal. There may be a case where the polarized-waves have optical power difference because of variation of characteristics between each modulator. And so, U.S. Pat. No. 6,819,872 (hereinafter referred to as Document 1) discloses a feed-back control of optical power of each polarized-wave with use of a light-receiving element according to each polarized-wave.
However, the art of Document 1 detects optical power of each polarized-wave with use of two light-receiving elements. In this case, an initial adjustment is needed for reducing characteristics variation of the light-receiving elements and optical power difference appearing after an optical power controller. And, the art fails to compensate for aging fluctuation of the characteristics of the light-receiving elements and the optical power difference between each polarized-wave caused by aging fluctuation of optical combiner after the optical power controller.